The present invention generally relates to an assembly unit including a component and at least one thread forming screw. The component includes a through hole for each screw, and each screw includes a head serving for actuation of the screw, a supporting surface facing said component and a shank at least partly including a thread.
In the field of assembly technology, it is increasingly desired to use assembly units in which a plurality of elements has been pre-assembled to form a pre-assembly unit. In the following, the assembly unit including a component and usually a plurality of screws is connected to a work piece. A typical example of this technology is an upper part and a lower part which have to be interconnected by one or more screws. In such a case, the upper part as the component and the screws form a pre-assembled assembly unit, the screws being captively connected to the component. The assembly unit is then connected to the lower part by screwing the screws into the lower part. The screws may be simultaneously screwed into the lower part, or they may be screwed into the lower part one after the other. Especially, multispindle automatic screwing apparatuses are used for this purpose.
A typical field of use of such assembly units is the automobile industry. For example, a cylinder head cover or a cover of a transmission is the component which is to be connected to the work piece by screws. The work piece is the cylinder block or the housing of the transmission. By using such assembly units, a great level of prefabrication is realized. The screws are already connected to the component in a captive and aligned position. They are prevented from being damaged during transportation, and they may be quickly connected to the work piece.
An assembly unit is known from International Application PCT/EP95/00354 published as WO 95/21335 corresponding to U.S. Pat. No. 6,309,156. The known assembly unit includes a component, for example a cover of a housing, and at least one fixing element in the form of a screw. Each screw is associated with a third element, namely a bush being used for captive connection of the screw to the component. The bush is made of metal, especially of sheet metal. The bush increases the production costs of the known assembly unit. During a separate step of pre-assembly, the bush is pressed into a respective through hole being located in the component. The screw has a rather complicated shape. An annular flange is located in the region of the shank of the screw. The annular flange includes annular channels being connected to both sides. The annular flange has to be produced at the shank of the screw in addition to the thread. The size of the annular flange is coordinated with the size of a collar being located at the bush such that there an axial lock is realized, the axial lock still allowing for an axial movement within certain limits. Consequently, the screws are arranged to be captively connected to the work piece, but they are not fixedly connected to the work piece in the pre-assembled position. There also is the possibility of not using an annular flange at the screw. In such a case, special elements have to be located at the bush, these elements in connection with the screw forming an axial lock. Consequently, the bush has a more complicated design. In this case, the thread of the screw may fulfill a part of the function of the axial lock. There is a relationship between the diameter of the through hole in the component and the diameter of the shank of the screw only as far as the screw together with the bush has to be moved through the through hole, and it has to be inserted into the through hole, respectively. However, there is no exact axial alignment of the screw at the component since the screw is movable to a limited extent. This fact may be advantageous for screwing the screw into the work piece if it is ensured that the screw contacts the entrance into the thread into the work piece in a centering way. Due to the danger of tilting effects of the axes of the screw with respect to one another, inserting the free ends of the shanks of the screws into the threads in the work piece is difficult. This disadvantage may be counteracted by the bush as the third element in the known assembly unit having a comparatively great axial length. Especially, the length may be substantially more than the wall thickness of the component. Due to the increase of the length of the bush, the length of the screw also has to be increased. This increase means additional cost expenditure, and it may also have a negative influence on the function in some cases. The additional use of such bushes, meaning using assembly units including at least three separate elements, is disadvantageous as the bush is also clamped when the screw is screwed into the work piece. Especially in the case when a bush being made of comparatively thin deformable sheet metal is used, there is the danger of different deformations of the bushes occurring during tightening of the screws such that it is difficult to apply a reproducible pretension force.
A component being made of plastic to be screwed to a work piece with an intermediate arrangement of a seal is known from German Patent Application No. 196 51 838 A1. A bush is used in a way that it extends through the component. The bush is connected to the seal. The screw has to have an annular flange to be produced in a separate step, the annular flange cooperating with the bush in the sense of an axial lock. In this way, the screw is connected to the bush in a loose but captive way. In addition to the component and the screw, the bush is the third element of the known assembly unit. The bush does not necessarily have to extend through the entire through hole being located in the component. German Patent No. 195 46 072 C2 shows a bush having a shorter axial length and including a conical collar. The collar engages a respective through hole being located in the component and having a partly conical design. The collar only partly engages the through hole under the influence of frictional clamping effects. The captive arrangement is achieved by frictional forces such that there is substantial insecurity. The bush having a disc-like design finally is located between the supporting surface being located at the head of the screw and the respective counter surface being located at the component. The bush is clamped, and there are similar disadvantages as it has been described above.
Furthermore, pre-assembled assembly units also including at least three elements are known, the above described bush being replaced by a clip, a holding clamp or the like. The third elements means additional expenditure which often also has a disadvantageous effect on the function.
The present invention relates to an assembly unit including a component and at least one screw. The component includes at least one through hole. The screw is associated with the through hole of the component. The screw is designed and arranged to produce a thread. The screw includes a head and a supporting surface to face the component. The screw includes a shank at least partly including a thread. The screw in the region of the shank and close to the supporting surface includes an annular channel, the diameter of the annular channel being less than the core diameter of the thread of the screw and the effective axial length of the annular channel being more than the axial length of the through hole. The diameter of the through hole of the component is more than the core diameter of the thread of the screw and being less than the outer diameter of the thread of the screw.
The novel pre-assembled assembly unit does not require the use of a third element, as for example a bush, a clip element, a clamp element or the like. The screw and the screws, respectively, is captively and fixedly connected to the component in the pre-assembled position of the assembly unit.
The novel assembly unit includes one or more screws being designed to produce a thread. Such a thread forming screw is to be understood as a screw producing a thread without producing chips when being screwed into a respective bore not yet including a thread. Such screws may be also called thread rolling screws or thread producing screws. There is a certain relationship between the diameter of the through hole being located in the component and the size of the screw, especially in the region of its shank where the thread is located. In this way, the screw extends into the through hole, and it engages the through hole, respectively, during pre-assembly. Consequently, there is not only an axial lock, but also a fixed aligning relationship of the axis of the screw with respect to the component. The alignment of the axis of the screw is determined, and the screw can no longer tilt. When the screw only extends into the through hole but not through it, the thread of the screw is partly located in the region of the through hole and partly at the side of the screw outside of the through hole facing the head of the screw. Consequently, the thread is located in a protected position during transport from the manufacturer of the pre-assembled assembly unit to the place where the assembly unit is used. Thus, the screw will not be damaged during transportation.
Despite the fact of the improved function of the fixed position and the aligned axis in which the screw is captively arranged, there is no need for using a third element, for example a bush, a clip and the like. Consequently, production expenditure is respectively decreased. The expenditure is further decreased by using self forming screws the threads of which may also be used to attain the axial lock. Consequently, there is no need for arranging a separate annular flange at the shank of the screw.
Especially when the component includes a plurality of such screws in the pre-assembled position such that it is designed for multiple screwing connections to the work piece, it is preferred that the screw in the region of the shank between the supporting surface at the head and the threaded portion includes an annular channel forming an undercut. The diameter of the annular channel is less than the core diameter of the thread, and the axial length of the annular channel (taking the respective transition radiuses into account) is more than the axial length of the through hole being located in the component. Due to the chosen design of the thread forming screw, an axial locking and alignment arrangement, on the one hand, and a captive arrangement of the screw, on the other hand, is realized. Additionally, the screw and the screws, respectively, in the region of the annular channel forming the undercut do not contact the thread being produced in the component in a radial direction and in an axial direction. Consequently, taking the tolerances into account, adaptation of the screw and of the screws, respectively, is possible. As soon as the screw with its thread has exited the thread being located in the through hole of the component, the component may adapt with respect to the work piece to a limited extent in an axial and/or a radial direction. When further tightening the screw, the pre-tension force is applied. When using a plurality of screws, possible initial tensioning of the component at the work piece is released by the screws when the threads of the screw exit from the component such that the component reaches a free relative position with respect to the work piece. This adaptation may be used when the screws are screwed into the work piece one after the other. The same applies when a plurality of screws or all screws are screwed into the work piece. When all screws are screwed into the work piece, it is especially advantageous when the bores being located in the work piece into which the screws are to engage do not include threads. Instead, they are bores which have been produced simply by casting or by cutting. In this way, it is not necessary to initially produce threads in the work piece, and the thread forming properties of the screw are used twice. Mutual tensioning of the component with respect to a plurality of screws is prevented since each screw produces its own thread in the bore being located in the work piece. In this way, different angle positions of the threads of the screws are automatically taken into account when screwing in the screws. When meeting respective tolerances, it is also possible to connect the pre-assembled assembly unit consisting of the component and a number of screws with a work piece the bores of which already including threads. This applies to simultaneously screwing a plurality of screws into the work piece as well as to screwing a plurality of screws one after the other.
Each thread being located at the screws may include a joining portion or a connection portion facing away from the head of the screw. The outer diameter of the thread in the joining portion is less than the diameter of the through hole being located in the component. The mutual adaptation is chosen in a way that the screw with the joining portion may be introduced into the through hole and the thread forming function of the thread of the screw is used when screwing the screw into the component. Each thread being located at the screws may include a joining portion, a thread producing channel portion and a smoothening portion. The axial length of the joining portion and of the groove portion are approximately identical to or less than the axial length of the through hole being located in the component. In this case, there is the possibility that each screw does not extend beyond the through hole or only extends beyond the through hole to a small extent in the pre-assembled position of the screw. Such an arrangement may be advantageous when it is desired to realize great safety with respect to damages of the thread of the screw and when a centering function of the free ends of the screws when being connected to the work piece is not of great importance.
On the other hand, there also is the possibility of the thread of the screw including a joining portion, a groove portion being designed and arranged to produce a thread without removing chips and a smoothening portion, the axial length of the joining portion and of the groove portion being more than the axial length of the through hole being located in the component. In such a case, the screw also has a centering function.
In all cases, the shank of the screw in the region of the thread may have a trilobulare cross-section. Such a trilobulare cross-section is a so called an xe2x80x9corbiformxe2x80x9d element. This means that it has the same diameter at all positions about the circumference of the cross-section when measuring at distances of 180xc2x0, although the cross-section approximately has a triangular shape. Such a trilobulare cross-section also fulfills the function of self-centering effects, and it also allows for forming the thread in the component and possibly also in the work piece without producing chip.
It is especially advantageous when the through hole being located in the component has a diameter which is more than the flank diameter and less than approximately 90% of the outer diameter of the thread being located at the shank of the screw. The diameter of the through hole is located in the range being defined by the two limits. When keeping these limits, resistance during forming of the thread is sufficiently small. In other cases, there would be a weakly formed thread such that the screws cannot be held in the thread as desired. When keeping the described limits, the necessary pre-tensional force may be applied properly and reproducibly after screwing.
For example, the through hole may be located at the edge of a cover-like component, the edge with its supporting surface either directly contacting the work piece or contacting the work piece with an intermediate arrangement of a seal or a different element. It is also possible that the component includes a supporting surface for combination with the work piece and that the through hole being located in the component is located in an axial rear direction with respect to the supporting surface. Especially when using components having comparatively thin walls, it is possible to design a hat-like impression, especially by deep-drawing, the impression being located in a backward direction with respect to the work piece in the connection direction such that the screw in the pre-assembled position is protected to an increased extent. Since the screw is often made of a material of greater strength than the work piece, there is no actual danger of the surface of the work piece being damaged. In this exemplary embodiment, the length of the screw is increased. However, this is desired for some applications, especially when the screw has a stretching portion of respective axial length.
The connection portion of the thread may be located at the free end of the shank of the screw, and it may begin at the free end of the screw, respectively. On the other hand, it is also possible that the screw at the free end of its shank has a centering portion not including a thread. Such a centering portion protects the thread from being damaged, and it simplifies centering of the component at the work piece by the screws.
Other features and advantages of the present invention will become apparent to one with skill in the art upon examination of the following drawings and the detailed description. It is intended that all such additional features and advantages be included herein within the scope of the present invention, as defined by the claims.